In conventional phase shift modulators the modulated signal must be spectrum shaped before it is transmitted in order to reduce adjacent channel interference. The phase modulation is typically carried out at a fixed frequency. After the phase modulation the modulated signal is bandpass filtered to shape its spectrum. The modulated signal is then translated up to the transmit frequency. The modulated carrier at the transmit frequency is then passed through another bandpass filter centered at the transmit frequency. This conventional modulation technique requires expensive and bulky bandpass filters as well as a frequency translator.
In view of the above conventional phase modulation technique, there exists a need for a method and apparatus to carry out phase modulation in such a way as to eliminate the need for the bandpass filters and the frequency translator.